Composite panels are widely used as flooring, walls and partition materials in the construction of airplanes due to such composite panels' lightness of weight and bending resistance.
Typically, composite panels comprise a thick sheet of honeycomb core sandwiched between a pair of facing sheets. The honeycomb core is commonly made from an aluminum foil or a paper material and comprises an array of closely-packed hollow channels which are typically hexagonal, square or circular in shape.
The honeycomb core is often filled with a foam material such as polyurethane foam to achieve enhanced physical properties. The hollow structure of a honeycomb core typically accommodates significant amounts of air through which heat and noise can transmit across the panel thickness. Thus, filling the cells of the honeycomb core with foam improves the thermal and acoustic insulation of the composite panel. Elimination of hollow spaces within the honeycomb core also reduces the chances of moisture accumulation and internal corrosion. Furthermore, to various extents, depending on the foam filling density, the filled honeycomb core has higher mechanical strength against compressive, shear and impact forces than does a similar, unfilled honeycomb core.
In some methods of the prior art, the honeycomb core is pre-filled with foam prior to the assembly of the honeycomb core between the facing sheets. In related prior art methods, a thick honeycomb core block (from which multiple honeycomb cores can be cut) is pre-filled with foam prior to the block being cut up into individual honeycomb cores. Unfortunately, such a method of pre-filling the honeycomb core with foam typically results in uneven foam density along the height of the honeycomb cells, i.e., the direction of foam growth inside the honeycomb cell. Moreover, a slice of honeycomb pre-filled with foam is frequently found to be difficult to bond to, because foam debris easily contaminates the honeycomb cell edge, causing weak adhesion.
In other core foaming methods, dry particulate foam precursors are disposed loosely within the cells of the honeycomb core. After a panel precursor is assembled, the panel precursor is heated to cause the particulate foam precursors to create a foam within the cells of the honeycomb core. Unfortunately, in the deposition process, it is difficult to uniformly distribute the particulates over all the honeycomb cells. As a result, the quality of the resulting foam is inconsistent. Moreover, the method of using dry particulate foam precursor materials cannot be adapted for use in a thick honeycomb core block. Still further, operations involving a large quantity of dry particulates are unsafe and a nuisance to operations personnel.
Finally, the foam filling methods of the prior art are typically unable to deliver foam densities lower than about 2 pounds per cubic foot. This is unfortunate because, in applications where thermal and/or acoustic insulation is desired, low density foam filling (preferably lower than 1 pounds per cubic foot) would be ideal.
Accordingly, there is a need for a new method of manufacturing a foam-filled composite panel which avoids the above-described problems in the prior art.